1. Field of the Invention
The present invention is directed to a signal conditioning circuit particularly for use with data acquisition equipment such as data loggers or other automated data acquisition equipment as well as other types of industrial controls and sensors. The signal conditioning circuit has particular application to intrinsically safe circuits used for data acquisition in hazardous environments.
More specifically, the invention relates in to an Op Amp circuit, particularly for use in signal conditioning devices. In signal conditioning circuits, it is often necessary to adjust both the offset (zero) and gain (span) of a conditioning amplifier to accommodate widely varying input sensor signal ranges from various types of sensors (i.e., thermocouple, thermopile, thermistor, platinum RTD, pressure transducer, etc.) and provide a normalized output signal such as the standard 4-20 mA current output used in process control instrumentation.
2. Description of the Prior Art
Prior art devices solved this problem in one of two ways. The first was to provide a separate conditioning amplifier circuit for each type of signal input, typically mounted on a printed circuit board "cards" which could then be interchanged with other similar "cards". This approach, of course is rather costly, as an inventory of different signal conditioning circuit cards must be kept for each type of input. An alternative to this approach is to provide a generic signal conditioning card which can be easily field modified to provide appropriate signal conditioning for a variety of inputs. The use of a generic signal conditioning circuit greatly reduces inventory costs and results in a much more flexible design, as the user need not purchase a plurality of separate signal conditioning cards for each type of input.
In order to provide such a generic signal conditioning circuit, broad offset limits (Zero) and gains (Span) must be provided to insure the circuit will function for a wide variety of inputs. Typically, the zero offset must be able to provide for an adjustment ranging from -130% to +130% of full scale and a span adjustment range of 100% to 260% of full scale with a minimum step size of 10%. The most intuitive approach to such a problem would be to provide a plurality of biasing resistors for adjusting the zero and span of the signal conditioning circuit, with a switching device to allow the appropriate resistor to be placed in the circuit. Unfortunately, in order to provide the appropriate ranges discussed above, as many as 26 resistors would be needed for each of the zero and span adjustments using such a "binary" approach.
In fact such circuits have been used that combine a plurality of resistors having a binary ohmic value sequence to enable the measuring range to be calibrated according to a two state (on/off) criterion, which is a binary system. Such circuits, however, do not ensure, within the above measuring range, appropriate values and amounts of resistors and jumpers.
More particularly, for the regulation of the so-called start scale value (Zero) and for the calibration of the gain (Span), nine resistors and ten jumpers are, for example, needed in order to have five position steps (i.e., five reference steps of the measuring range).